Samsung Galaxy S4 mini — Old email reappearing as unread

Samsung Galaxy S4 Mini: I have linked my email account successfully, it works just fine. On the other hand, I am constantly getting notification/notifications that I got new email/emails, but actually an old one is added at the bottom of the list of 25 emails and marked as unread. I mark it "seen/read", but after several minutes I get again notification for a new email, but again the old one at the bottom, that I already marked as "seen/read", reappears as the new message and there is nothing I can do about that, even though I carefully checked out all settings' options.

It's really annoying as I think I've got new emails when I haven't. The same email is marked as "seen/read" on my computer.